EGG
Genetic mutations occur during the creation of the cells that will ultimately be the new lifeform, since this can only occur once a females eggs have been fertilized and the new life starts to become what ever the gene sequence has shaped it to become, yet sometimes mutatations occur at this stage which can affect how the lifeform develops throughout its life.
If this mutation is to the advantage of the lifeform then there's a good chance it will reproduce with other lifeforms of the same species until this dominant gene is more widespread. But what we also have to remember it's not only genes that shape lifeforms but the enviroment in which it lives.
So for instance, you could get 2 newly formed slighty different chickens, one lives in a region where there are alot of ground predators, and due to this, this chicken starts to use its wings more and more, and if somehow the species survived long enough, I'm sure they'd learn to fly again. Where as the other chicken gets captured by an ape and put in cages all its life, eventually due to the internal heating of the hatcheries feathers are no longer needed to keep the chicken warm so it starts to lose its feathers. After hundreds of thousands of years, each chicken would continously go on a different path of evolution, partly down to enviroment and partly down to isolation from one another.
In the end we will have 2 different species of chicken and maybe in many many more years, 2 species of bird.
But it started with an egg!
Poll